The binding specificity of rhesus antibodies elicited by various conjugate vaccines of group B Neisseria meningitidis (GBNM) was studied using a variety of group B meningococcal polysaccharide (GBPS) antigen systems. The antibody response, as measured by ELISA, was significantly higher to the spacer-containing GBPS or N-propionylated version of GBPS than to the native GBPS complexed with methylated human serum albumin (HSA). The specificity of a population of antibodies directed to the spacer, ADH, was confirmed by their binding to an unrelated pneumococcal 9V polysaccharide conjugated to HSA through ADH. Antibodies elicited by GBPS-OMV conjugate vaccine also showed binding to N-propionylated GBPS conjugated to HSA. As an extension of the previous immunization schedule, four groups of monkeys were further immunized with the purified GBPS alone at week 50. There was no booster response to the pure GBPS. The sera were examined for bactericidal antibodies. Using human complement, the vaccines induced no increases in bactericidal antibody directed against group B polysaccharide determinants. However, the conjugates containing outer membrane vesicles, but not the unconjugated vesicle control vaccine induced high levels of bactericidal antibodies directed at lipopolysaccharide determinants.